1. Field of the Invention
The present invention relates to backlight units and, more particularly, to a backlight unit having a light guide plate that illuminates liquid crystal displays provided at both sides thereof.
2. Description of the Background Art
Liquid crystal displays generally use a light source known as a “backlight unit” because the liquid crystal is not self-luminous. Fold type cellular phones, which are now becoming the main trend, need to illuminate two liquid crystal displays provided on the front and back sides thereof. Under these circumstances, various techniques have been proposed for backlight units to illuminate both the front and back liquid crystal displays with a single light guide plate for the purpose of space and cost savings (this type of backlight unit will be hereinafter referred to as a “double-sided backlight unit”). Japanese Patent Application Publication No. 2002-189230, for example, proposes a double-sided backlight unit wherein a single light guide plate is provided between liquid crystal displays spaced from each other, and sheets selectively transmitting either p-polarized light (p-component of light) or s-polarized light (s-component) and reflecting the light that is not transmitted are provided between the light guide plate and the two liquid crystal displays, thereby efficiently transmitting light from the light guide plate to the liquid crystal displays. The proposed technique will be explained below with reference to FIGS. 7A and 7B.
FIG. 7A shows the relationship between a liquid crystal display panel and illuminating light emitted from a light guide plate in a conventional backlight unit. FIG. 7B shows the relationship between a liquid crystal display panel and illuminating light emitted from a light guide plate in the backlight unit of the above-described conventional art.
In the conventional backlight unit, as shown in FIG. 7A, p-component, or p-polarized light 56, and s-component, or s-polarized light 58, are emitted from a light guide plate 10 toward a liquid crystal display panel 25. Because upper-side and lower-side polarizing plates 52, 54 of the liquid crystal display panel 25 is of the absorption type, the s-polarized light 58, for example, is absorbed, and only the p-polarized light 56 illuminates a liquid crystal layer-retaining portion 50, and p-polarized light 60 passes through white-display regions (i.e. activated liquid crystal cells) of the liquid crystal display panel 25.
In FIG. 7B, a reflective polarizing sheet 20 is provided between a liquid crystal display panel 25 and a light guide plate 10. Let us assume, for example, that the reflective polarizing sheet 20 is of the type that reflects s-polarized light but transmits p-polarized light. Of p-polarized light 56 and s-polarized light 58 emitted from the light guide plate 10, only the p-polarized light 56 is transmitted through the reflective polarizing sheet 20. The s-polarized light 58 is reflected by the reflective polarizing sheet 20 to become s-polarized light 62 returning to the light guide plate 10. The p-polarized light 56 transmitted through the reflective polarizing sheet 20 becomes p-polarized light 60 that passes through white-display regions of the liquid crystal display panel 25 in the same way as in FIG. 7A.
The s-polarized light 62 returned to the light guide plate 10 is modified into p-polarized light and s-polarized light in the light guide plate 10 so that p-polarized light and s-polarized light are emitted toward the reflective polarizing sheet 20 as denoted by 64 and 66, respectively.
Thus, if the type of polarized light (p-polarized light or s-polarized light) absorbed by the lower-side polarizing plate 54 of the liquid crystal display panel 25 is the same as type of polarized light that is reflected by the reflective polarizing sheet 20, there is no light absorption by the lower-side polarizing plate 54 of the liquid crystal display panel 25. Consequently, the light utilization efficiency increases to a considerable extent.
In the case of a double-sided backlight unit in which a single light guide plate illuminates first and second liquid crystal displays disposed at both sides thereof, the light utilization efficiency can be increased by setting the type of polarized light (s-polarized light or p-polarized light) absorbed by the polarizing plate of the first liquid crystal display panel to be the same as the type of polarized light reflected by a first reflective polarizing sheet provided between the first liquid crystal display panel and the light guide plate, and setting the type of polarized light absorbed by the polarizing plate of the second liquid crystal display panel to be the same as the type of polarized light reflected by a second reflective polarizing sheet provided between the second liquid crystal display panel and the light guide plate and, at the same time, different from the type of polarized light reflected by the first reflective polarizing sheet.
Such a reflective polarizing sheet is commercially available, for example, as “RDF series” from Sumitomo 3M Ltd.
The foregoing is the fundamental part of the technique disclosed in the above-mentioned application. In the double-sided backlight unit, however, if the two liquid crystal display panels of the liquid crystal display device are different in display area or size from each other, some problems arise.
The first problem is that the use of two reflective polarizing sheets causes an increase in cost. The second problem is brightness unevenness occurring on the one of the two liquid crystal display panels that is larger than the other, as will be explained below.
The problem of brightness unevenness will be explained below by using a liquid crystal display device shown in FIG. 8.
In the liquid crystal display device, a double-sided backlight unit having a light source 12, a light guide plate 10, and reflective polarizing sheets 20 and 21 is held in a housing 40. A liquid crystal display panel 26 having a small display area and a liquid crystal display panel 24 having a large display area are set in respective apertures 41 and 43 formed in the housing 40.
It is herein assumed that the reflective polarizing sheet 20 is of the type that reflects s-polarized light but transmits p-polarized light; the reflective polarizing sheet 21 is of the type that reflects p-polarized light but transmits s-polarized light; the light guide plate-side polarizing plate (not shown) of the liquid crystal display panel 26 is of the type that absorbs s-polarized light; and the light guide plate-side polarizing plate (not shown) of the liquid crystal display panel 24 is of the type that absorbs p-polarized light.
Of p-polarized light 56 and s-polarized light 58 emitted upward from the light guide plate 10, only the p-polarized light 56 is transmitted through the reflective polarizing sheet 20. The s-polarized light 58 is reflected by the reflective polarizing sheet 20 to become s-polarized light 62 returning to the light guide plate 10. The p-polarized light 56 transmitted through the reflective polarizing sheet 20 passes through white-display regions of the liquid crystal display panel 26. Of s-polarized light 57 and p-polarized light 59 emitted downward from the light guide plate 10, only the s-polarized light 57 is transmitted through the reflective polarizing sheet 21. The p-polarized light 59 is reflected by the reflective polarizing sheet 21 to become p-polarized light 63 returning to the light guide plate 10. The s-polarized light 57 transmitted through the reflective polarizing sheet 21 passes through white-display regions of the liquid crystal display panel 24.
P-polarized light 72 that is transmitted through the reflective polarizing sheet 20 and impinges on the inner surface of the housing 40 (where the liquid crystal display panel 26 is not present) is reflected by the housing inner surface to become p-polarized light 80, which is then transmitted through the reflective polarizing sheet 20 to enter the light guide plate 10. The p-polarized light 80 is modified into p-polarized light and s-polarized light in the part of the light guide plate 10 that the p-polarized light 80 entered. Accordingly, s-polarized light 73 and p-polarized light 75 emitted downward from that part of the light guide plate 10 increase in light quantity. Although the p-polarized light 75 is reflected by the reflective polarizing sheet 21, the s-polarized light 73 having an increased light quantity is transmitted through the reflective polarizing sheet 21 and passes through white-display regions of the liquid crystal display panel 24. Accordingly, the part of the liquid crystal display panel 24 that is illuminated by the s-polarized light 73 becomes brighter than the part S of the display panel 24 that is not illuminated by the s-polarized light 73 (i.e. the part facing the liquid crystal display panel 26). As a result, brightness unevenness occurs in the display area of the liquid crystal display panel 24.
The influence of the s-polarized light 73 can be eliminated by disposing a light-diffusing sheet having high diffusing capability between the light guide plate and the liquid crystal display panel of larger display area to resolve the problem of brightness unevenness. By doing so, however, the light utilization efficiency is reduced correspondingly.